Electrohydraulic valve systems typically include a pressure regulator that responds to electromagnetic forces to position a valve element against a valve spring force. In the case of a pressure regulator valve, the output pressure is determined by a transfer function for the electrical parameter (e.g., magnetizing current) and the mechanical force applied to the valve. Valve systems of this kind typically are characterized by a hysteresis due to the effect of friction caused by oil-borne contaminants and the magnetic hysteresis of the electromagnetic actuator. A major factor in determining the frictional hysteresis is the presence of oil-borne contaminants in the hydraulic fluid.
Hysteresis in the transfer function results in a variable output pressure for any given actuating current. For an electrohydraulic valve with a proportional transfer function, the output pressure obtained when the actuating current increases is less than the output pressure obtained for the same current when the current decreases. The output value for the pressure thus is not precisely proportional to a given input parameter or magnetizing current. The opposite is true for an inversely proportional electrohydraulic valve.
U.S. Pat. No. 4,577,143 describes an attempt to establish a more precise control of the position of a regulator valve for a given value of an input parameter so that a predictable characteristic transfer curve at any value for the input parameter can be determined. This is accomplished in the system described in the '143 patent by changing the current in a direction that is opposite to the direction of the transfer function curve. This effects a reversal of the magnetization to overcome the hysteresis gap between the magnetization curves. The reversal of the magnetization occurs only when a change in command is requested.
Electrical energy is applied only long enough to effect re-magnetization. The transduced value, which is in the same direction as the selected characteristic, will follow the selected magnetization curve. The excess current value is not applied long enough to cause a change in the mechanical output of the valve.